The present invention relates to light filters and more particularly to doped, high silica glasses for the production of such filters.
High silica glasses are glasses consolidated from a porous state and containing, exclusive of additives, at least about 94% silica. Such glasses, also referred to as reconstructed and 96% silica glasses, are similar to fused silica in their ability to resist softening at high temperatures. Of particular interest for many applications is the fact that high silica glasses have coefficients of thermal expansion (CTEs) on the order of 7-14.times.10-7/.degree. C. This property renders them resistant to heat shock.
The production of such glasses was first disclosed in U.S. Pat. Nos. 2,215,039 and 2,286,275 (Hood et al.). As there described, such glasses are melted as a precursor, phase-separable, borosilicate glass. This glass is then heat treated to separate the glass into a silica-rich matrix phase and a borate-rich second phase. The Latter phase is readily soluble, and may be removed by solution in a mineral acid such as nitric acid. The porous glass matrix that remains is largely silica. It may then be thermally consolidated to produce a non-porous, clear glass of lesser volume and having a silica content of at least 94%.
It was subsequently learned that porous, high silica glasses could be impregnated with solutions containing a variety of different metal salts. These salts convert to oxides during consolidation and produce a modified glass. The production of colored glasses in this manner is taught in U.S. Pat. Nos. 2,303,756, 2,340,013 and 2,355,746 (Nordberg et al.). Among the colors reported is a yellowish-green obtained with a chromium oxide addition.
Patents issued subsequent to the initial development of these processes expanded the range of colors and intensities available. For example, U.S. Pat. No. 3,188,217 (Elmer et al.) produces an amber-colored, color-correcting filter by incorporating a combination of iron, nickel and aluminum oxides in a high silica glass, while U.S. Pat. No. 3,188,218 (Elmer et al.) provides a blue, color-correcting filter by incorporating a combination of Co.sub.2 O.sub.3, P.sub.2 O.sub.5 and an alkali metal oxide in a high silica glass.
U.S. Pat. Nos. 3,258,631 and 3,399,043 (Elmer et al.) describe selectively impregnating porous glass tubing from the outside or external surface only. U.S. Pat. No. 4,073,579 (Deeg et al.) produces porous surface layers on ophthalmic lenses and impregnates them with various colorants to produce tinted lenses, although no association of color and colorant is made.
The above colorants and many others have been used to make colored glasses by conventional melting processes, although the colors obtained in melted glasses may differ substantially from those in reconstructed glasses depending upon factors such as the state of oxidation of the glass melt and interactions between colorants and other components of the molten glass. The text "Coloured Glasses", by W. A. Weyl, (1959), for example, describes the use of various chromium, cobalt, copper, iron, neodymium, sulfur, titanium and/or vanadium compounds to produce blue glasses. Chapter XII at pages 168-196 of that text deals particularly with the use of cobalt for the coloring of glass.
It has recently become desirable to provide a glass having a controlled blue color for external automotive lighting. However, with the advent of halide lamps, glasses used in lamp envelopes now must withstand greatly increased service temperatures. Thus ordinary lamp glasses cannot be used. As a consequence, a need has developed for a refractory high silica glass which would exhibit a specifically prescribed blue color.
First considered in addressing this need was the use of cobalt as a dopant for a porous high silica glass. Unfortunately, upon consolidation the resulting glasses exhibited colors more akin to lavender, pink or purple, a result attributed to the presence of red as well as blue color centers in the glass.
Accordingly, it is a basic purpose of the present invention to provide high silica glasses that meet the need for a purer blue glass for color filter applications. Another purpose is to provide a reliable method of developing the desired color in a high silica glass.